Television receiver



Sem.. 8, H42 C, N, SMYTH A 2,295,059

TELEVISION RECEIVER Filed Dec. 50, 1938 Atta/v1@ l Patented Sept. 8, 1942 TELEVISION RECEIVER Charles Norman Smyth, Si'dcup, Kent, England, assignor to International Standard Electric Corporation, New York, N. Y.

Application December 30, 1938, Serial No. 248,444 In Great Britain February 4, 1938 Claims.

This invention relates to television receivers and more particularly to arrangements for providing a control of the contrast in the reproduced picture. A

The contrast in a television receiver employing cathode ray scanning is controlled by varying the amplitude of the vision frequency voltages applied to the modulator grid of the cathode ray tube. According to the present invention this is effected by the provision of means for obtaining a variable degree of inverse of negative feedback from the output circuit to the input circuit of a vision frequency amplifier stage in the receiver, whereby the gain of the amplifier is controlled. This method of contrast control has the advantage that it does not disturb the frequency response of the television receiver to any noticeable extent.

If the signal voltages are rectified to restore the direct current component, a variation of the amplitude of the modulator voltages applied to the cathode ray tube also results in a variation in the magnitude of the direct current component, and a change in contrast is normally accompanied by an undesirable change in the mean brightness of the reproduced picture. An object of the invention is to overcome this objectionable eifect.

According to another feature of the invention, a voltage derived from an adjustable impedance which is employed to control the contrast of the reproduced picture is used in counteract variation in direct current component which otherwise tends to bring about a change in the mean picture brightness when the contrast control is adjusted. In a preferred arrangement in which a diode is connected across the output circuit of a vision frequency amplifier for restoring the direct current bias and in which contrast control is eected by variation of a feedback resistance, one end of the diode load resistance is connected to a tapping on a potentiometer across the feedback resistance or to a point on the feedback resistance itself so that variations in the direct current component consequent upon variation of the degree of feedback are compensated.

The accompanying drawing shows schematically suicient of the output stage of a television receiver for a proper appreciation of the invention.

V is the vision output pentode valve the control grid of which is coupled to the preceding detector, by a coupling condenser Cl and its associated resistance Rl. The anode of the valve V is coupled to the modulating grid G of the cathode ray tube CRT over a condenser C3. R2 is a bias resistance and C2 its associated bypass condenser. A variable resistance R3 is included in the cathode lead of the valve V. This resistance is common to the input and output circuits of the valve and therefore provides inverse or negative feedback the amount of which may be varied by Varying the resistance R3. This adjustment will vary the gain of the output stage and thereby provides a means for varying the picture contrast.

D is a diode valve and R6 its associated load resistance. The cathode of the diode and one end of resistance R6 are connected to the modulator grid G of the cathode ray tube CRT, whilst the anode of the diode and the other end of the resistance R6 are connected to the junction of two resistances R4 and R5 connected across the feedback resistance R3. 'I'he steady grid bias on the grid G of the cathode ray tube CRT is tapped off a potentiometer R1 connected across a source of high tension. The potentiometer R1 thus provides a control for the mean picture brightness.

The diode D restores the direct current compo-` nent in the input circuit of the cathode ray tube. Vision frequency signals are applied across the diode. The potential across the diode load resistance R6 and consequently the bias of the grid G, depend upon the mean amplitude of the signals and the mean illumination of the picture at the transmitter. 'I'he brightness of the received picture will therefore vary in accordance with changes in the mean illumination at the transmltter.

In operation, increase of the feedback resistance R3 increases the amount of negative feedback and reduces the gain of the output amplifier stage. The vision frequency voltages applied to the modulator grid of the cathode ray tube are thus reduced and the picture becomes less contrasting. The voltages applied to the direct current restoring diode D are also reduced and if the lower end of the load resistance R6 were connected in normal manner directly to the grounded bus-bar this would result in a reduction in the mean brightness of the reproduced picture. The reverse eect will occur when the feedback resistance is decreased. This objection is overcome by connecting the anode of the diode and the lower end of the load resistance R6 to a tapping on a potentiometer constituted by the resistances Rt and R5 across the feedback resistance R3. By a suitable choice of resistances Rt, R5 the increase or decrease in direct current voltage across the feedback resistance may be made to compensate for the accompanying reduction or increase in the output from the diode across the load resistance RS and unwanted variations in the mean picture brightness thereby counteracted. An approximate correction can be obtained by dispensing with resistances R4 and R5 and connecting the anode of the diode D and the lower end of resistance R6 to the upper end of the feedback resistance R3.

What is claimed is:

l. A television receiver comprising a cathode ray tube for picture reproduction having a control grid and a modulating circuit connected thereto, a vision frequency amplifier for amplifying picture signals and for supplying current to said circuit, said amplifier having negative feedback means between its input and output circuits for feeding back picture signal energy over an appreciable range of picture signal frequencies, and means for varying the degree of such feedback for controlling contrast in the reproduced picture.

2. A device in accordance with claim l wherein said negative feedback means comprises an adjustable resistance common to the input and output circuits of said amplifier.

3. A television receiver comprising a cathode ray tube for picture reproduction having a control grid and a modulating circuit connected thereto, a vision frequency amplier for supplying current thereto, an adjustable resistance common to the input and output circuits of said amplifier for varying the degree of negative feedback in said amplifier for controlling contrast in the reproaaaaosa duced picture, means for rectifying vision frequency signals in the output of said amplifier to restore the direct current component of the signal current and means comprising circuit connections to said adjustable resistance for deriving from the latter a voltage to counteract variation in said direct current component, which variation would otherwise bring about a change in the mean picture brightness when the said adjustable resistance is varied.

4. A device in accordance with claim 3 wherein said means for deriving a voltage to counteracty variation in said direct current component com.- prises a potentiometer connected to said adjustable resistance and means to connect said potentiometer to said rectiiying means.

5. A television receiver comprising an ampliier comprising a tube having a cathode, an anode and a controlelectrode, an input circuit connected between said control electrode and said cathode, an output circuit connected between said anode and said cathode, feedback means common to said input and output circuits comprising an adjustable resistance in series with said cathode, compensating means comprising a potentiometer connected in parallel with said resistance, brightness restoring means comprising an impedance and rectifying means connected in parallel therewith connected to said output circuit and a point on said potentiometer, a cathode ray tube having control means for said ray and means for connecting said control means to said restoring means.

CHARLES NORMAN SMY'IH. 

